1. Field of the Invention
Embodiments of the present invention generally relate to the field of imagers made in monolithic form. More specifically, embodiments of the present invention relate to the structure of photodetectors used in such imagers.
2. Discussion of the Related Art
Fixed or mobile image acquisition devices are increasingly used in many fields. They must be inserted into smaller and smaller spaces. For example, image acquisition devices are inserted into portable phones. According to another example, in the medical field, it is desirable to have image acquisition devices of small dimensions able to be arranged on endoscopes.
It has thus been attempted to make, in monolithic form, image acquisition devices of small dimensions with an image quality at least comparable to that of known optical devices.
A monolithic image acquisition device includes photodetectors arranged at the intersection of lines and of columns of an array.
A cross-sectional view of FIG. 1 illustrates four photodetectors of a line or column of an image acquisition device. A photodetector comprises a photodiode D formed in a semiconductor substrate 1. The surface area taken up by photodiode D typically has a side or a diameter ranging between 0.2 and 1.5 μm. Substantially above the interval separating two neighboring photodiodes D, metal interconnects are formed in a dielectric 5. The metal interconnects are formed of several levels of metallization 3 and of vias. The thickness of dielectric 5 is generally greater than the total height of metallizations 3 and depends on technological constraints linked to standard technological processes and/or to the circuits formed around the image acquisition area, such as circuits for reading and processing the acquired images. The thickness of dielectric 5 typically ranges between 1.5 and 6.5 μm.
A succession of filters F of transparent resin colored in red R, green V, or blue B is formed in dielectric 5. Filters F follow one another so that, on a same line, two different colors alternate and that, on two successive lines, one color is common. For example, a first line comprises filters according to the sequence B-V-B-V and the next line comprises filters according to the illustrated sequence R-V-R-V. Filters F follow one another in a continuous fashion and the interface between two neighboring filters F is substantially above the middle of the interval separating two underlying photodiodes D. Thus, each photodetector comprises a filter F associated with a photodiode D. The filter F of each photodetector is topped with a respective converging lens L, also made of transparent resin.
It has been provided to form in each photodetector, between a filter F and its corresponding photodiode D, a waveguide G. Waveguide G is formed of a waveguide block 7 surrounded with dielectric 5. Block 7 is formed of a material with a refraction index n7 greater than that, n5, of dielectric 5 (n7>n5). Block 7 is of straight or slightly conical cylindrical shape. Block 7 is placed above lens L to receive the photons injected by lens L towards photodiode D. The high portion of block 7 is separated from filter F by a thickness of dielectric 5 negligible as to the induced light losses, typically on the order of a few nanometers. Similarly, the bottom of block 7 is placed above photodiode D and is separated therefrom by a thickness of dielectric 5 on the order of a few nanometers. The presence of waveguide G enables decreasing light intensity losses and avoiding wrong detections linked to a dispersion of the photons and/or to their refraction against the metallizations 3 which appear across the relatively significant thickness of dielectric 5.
According to one example of the waveguide G, the guide is formed of a cone of a silicon, oxygen, carbon, and nitrogen compound called silicon oxynitride, which exhibits an index n on the order of from 1.6 to 2.3 according to its stoichiometry formed in a thick silicon oxide layer (SiO2) of index n=1.43. Another example of the waveguide G uses tantalum oxide, which has an index n on the order of 2, as a high-index material.
However, the use of such materials may raise practical problems. In particular, block 7 of FIG. 1 of the present application is formed by filling a deep and narrow opening in the dielectric 5. “Deep and narrow” means in the present description an opening having a ratio between the depth (substantially equal to the thickness of dielectric 5) and the average diameter (substantially equal to that of photodiode D) greater than 2. The filling of such an opening, which is performed by chemical vapor deposition (CVD), must be homogeneous. However, on filling of a narrow and deep opening with compounds of silicon oxy-carbo-nitride type or with tantalum oxide, bubbles or cavities form. Such bubbles form traps for the received light Further, when block 7 is formed of a silicon oxy-carbo-nitride or of tantalum oxide, problems of mechanical hold with peripheral silicon oxide 5 can be observed. Moreover, some of the silicon oxy-carbo-nitrides, as well as the tantalum oxide, deteriorate during the thermal cycles implemented in the rest of the process, especially the encapsulation and packaging anneals performed at temperatures from 300 to 400° C.
In another approach, a waveguide element is formed of alumina of index 1.63 or of silicon nitride (n=1.83) formed in a thick silicon oxide layer n=1.43. According to a variation of this approach, the high-index block 7 is silicon oxide n=1.43, formed in a thick dielectric layer 5 made of a material with a lower index such as an oxysilane of index 1.39.
The use of alumina or silicon nitride in a thick silicon oxide layer may exhibit disadvantages similar to those described previously for tantalum oxide or silicon oxy-carbo-nitrides.
The use of silicon oxide in oxysilane may also raise problems. In particular, this results in a significant modification of the materials used in the optical area with respect to the material present in the neighboring non-optical areas in which it is desirable to keep silicon oxide as an interlevel dielectric 5. This complicates and increases manufacturing costs. Further, the use in a thick layer of oxysilane with a refraction index lower than that of silicon oxide raises problems of mechanical hold, of ability to be locally etched, especially to form metallization levels 3 and the vias, as well as problems of resistance to thermal stress, especially on forming of the metal levels.